<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=<st>, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
   <style>
       /* margin -left:a */
   div{
       width:100px;
       height:100px;
       background-color:red;
       /* text-align:center;设置文本居中 */
       margin-left: auto;
       margin-right: auto;
   }
   </style>
</head>
<body>
    <div>

    </div>
</body>
</html>